In wireless and mobile communication applications a long stand-by and talk-time is desired. During operation of a mobile communication device the required output power of the mobile communication device depends on the distance between the device and the base station. If the distance to the base station is small the output power can be reduced to reduce power consumption and to increase the talk-time. On the other hand, a high output power is required if the distance to the next base station is long. The required output power, however, does not only depend on the distance between the mobile communication device and the base station but also on other factors such as the orientation of the device, specifically the orientation of the antenna with respect to the base station, or the obstruction of the mobile communication device by infrastructure such as buildings. The constantly varying conditions of the reception and transmission place high demands on the performance of mobile communication devices.
There has been proposed a plurality of power amplifiers which are suitable for mobile communication applications. For instance Doherty-amplifiers have been used. Other approaches use power splitters such as Wilkinson splitters to divide the input signal and to feed the split signals into small power amplifiers which can be deactivated when a small output power is desired. By-passing the power amplifier is another approach discussed for mobile communication applications.
For illustrative purposes reference is made to FIG. 14 showing a theoretical probability density distribution of the required output power for an IS-95 mobile communication system. As it becomes apparent from FIG. 14, a RF-power of about 0 dBm is required for most of the time with maximum power of up to 20 dBm required in peak situations.
A mobile communication device is designed to ensure communication in areas which are remote from the next base stations. To this end, the maximum output power of the RF-power amplifier of the mobile communication device is adapted for these extreme situations. However, the efficiency of a power amplifier significantly changes with the output power and has an optimum typically in a saturation mode. FIG. 15 illustrates a typical characteristic power curve and the efficiency dependency (power added efficiency—PAE) as a function of the input power. FIG. 15 shows that the power consumption is nonlinear with respect to the desired output power. Particularly at small power levels the efficiency is mainly determined by the quiescent current of the power amplifier which cannot be reduced further without preventing switching from a class A into a class B or class C amplifier mode. This also increases the non-linearity of the power amplifier. It is also desired to work in the so-called “back-off” range of the amplifier to ensure a linear response of the amplifier which is for instance evaluated on the basis of input amplitude to output amplitude distortion (AM/AM) or input amplitude to output phase relation (AM/PM).